Method and apparatus for reducing the infrared and radar signature of a vehicle

ABSTRACT

The present invention provides a lightweight structure for simultaneously providing a reduced infrared and radar Signature, while adding little or no weight to a vehicle. As such, the present invention allows for substantial improvements over prior systems. An example of the type of vehicle able to make use of the present invention is a military helicopter, but there is nothing within the spirit and scope of the present invention limiting it to any particular vehicle. The teachings of the present invention are useful with any vehicle for which a reduction in infrared emissions and microwave reflections is desired.

TECHNICAL FIELD

The present invention relates generally to methods of reducing theinfrared and radar signature of a vehicle, specifically to the use ofinsulative and absorptive materials to reduce the amount of infraredradiation being emitted, and the radar signals being reflected, fromcertain aspects of the vehicle.

DESCRIPTION OF THE PRIOR ART

Vehicles involved in military operations have a need to reduce theirvisibility to opposing forces. This need exists for all methods modernmilitary forces use to detect and target enemies. Examples of suchmethods include visual detection, audio detection, active and passiveradar, and infrared detection. This need to a void detection isespecially critical for aircraft, such as airplanes and helicopters,which have a high likelihood of being targeted by enemy air and groundforces using any and all of the above detection methods.

To the end of reducing the infrared signature of aircraft, a number ofmethods have been developed. These include the use of special exhaustducting and shrouding to reduce the exhaust heat signature, and theaddition of infrared insulative and absorptive materials on the outersurface of the aircraft. Although these methods can be very effectivewhen properly employed, each of these methods has drawbacks. In mostcases, the addition of infrared-insulative and infrared-absorptivematerials to the outer skin of the aircraft represents a significantaddition of weight to the aircraft and may interfere with theaerodynamics of the aircraft, reducing the performance and the range ofthe aircraft.

With respect to the goal of reducing the radar signature of an aircraft,both the shapes of the surfaces of the aircraft and the materials on thesurfaces of the aircraft can be optimized to reduce the radar signature.Unfortunately, additional radar-absorptive materials carry with themadditional weight, and shapes optimized for minimal radar signaturegenerally exhibit less-than ideal aerodynamic characteristics.

It is important to note that the current techniques for reducing thesignature for infrared and radar are generally mutually exclusive. Inmany cases, attempts to reduce the signature in one area of concernactually increases the signature in the other.

SUMMARY OF THE INVENTION

The present invention provides a lightweight structure for providing areduced infrared and radar signature while adding little or no weight toa vehicle. As such, the present invention allows for substantialimprovements over prior systems. An example of the type of vehicle ableto make use of the present invention is a military helicopter, but thereis nothing within the spirit and scope of the present invention limitingit to any particular vehicle. The teachings of the present invention areuseful with any vehicle for which a reduction in infrared emissions andmicrowave reflections is desired.

The present invention involves the use of a unique combination ofthermal insulators and radar-absorptive honeycomb in the composite skinof an aircraft. According to the present invention, an aerogel isintroduced into the individual cells of the honeycomb, which arenormally filled with air. In certain instances, the aerogel takes theplace of solid fillers.

Using the aerogel in combination with the radar-absorptive honeycomb inthe manner described herein, substantial improvements in the reductionof the aircraft signature can be realized with a negligible differencein the weight of the aircraft. Employed properly in a composite sandwicharrangement, the honeycomb can provide significant structural integrityto the outer surfaces of the aircraft. As such, the honeycomb is not“dead weight.”

Although aerogels are generally not employed for structural purposes,they have the distinct advantage of being extremely light in weight fora given volume. Furthermore, aerogels are extremely good insulators, sothat a relatively small volume, and therefore mass, of aerogels canprovide a substantial improvement in thermal performance.

One significant advantage of the present invention is that the infraredsignature and the radar signature of a vehicle can both be reducedsimultaneously, without causing adverse effects in either of these areasof concern.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the features and advantages of thepresent invention, reference is now made to the detailed description ofthe invention along with the accompanying figures in which correspondingnumerals in the different figures refer to corresponding parts and inwhich:

FIG. 1 is a perspective view of a radar-absorbing honeycomb panel inwhich the individual cells are fully filled with aerogel in accordancewith one embodiment of the present invention;

FIG. 2 is a close-up perspective view of the honeycomb panel of FIG. 1;

FIG. 3 is a perspective view of a radar-absorbing honeycomb panel inwhich the individual cells are half-filled with aerogel in accordancewith one embodiment of the present invention;

FIG. 4 is a close-up perspective view of the honeycomb panel of FIG. 3;

FIG. 5 is a perspective view of a radar-absorbing honeycomb panel inwhich the individual cells are half-filled with aerogel in accordancewith one embodiment of the present invention;

FIG. 6 is a close-up perspective view of the honeycomb panel of FIG. 5;

FIG. 7 is a perspective view of a laboratory testing station useful fortesting the thermal performance of the honeycomb panel of the presentinvention;

FIG. 8 is a close-up perspective view of the blackbody heat source andtemperature controller shown in FIG. 7;

FIG. 9 is a perspective view of the laboratory testing station of FIG. 7showing a honeycomb panel according to the present invention affixed tothe front of the blackbody heat source;

FIG. 10 is a close-up perspective view of the blackbody heat source andtemperature controller shown in FIG. 9, with the honeycomb panel affixedto the front of the blackbody heat source;

FIG. 11 is a computer screen shot of a thermal image of the front sideof the honeycomb panel opposite the blackbody heat source;

FIG. 12 is a set of computer screen shots of thermal images of the frontside of a honeycomb panel having no aerogel in its cells;

FIG. 13 is a set of computer screen shots of thermal images of the frontside of a honeycomb panel having its cells 25% filled with aerogel;

FIG. 14 is a set of computer screen shots of thermal images of the frontside of a honeycomb panel having its cells 50% filled with aerogel;

FIG. 15 is a set of computer screen shots of thermal images of the frontside of a honeycomb panel having its cells 75% filled with aerogel;

FIG. 16 is a set of computer screen shots of thermal images of the frontside of a honeycomb panel having its cells 100% filled with aerogel; and

FIG. 17 is a set of computer screen shots of thermal images of the frontside of a honeycomb panel having half of its cells 100% filled withaerogel and half of its cells empty.

DESCRIPTION OF THE PREFERRED EMBODIMENT

While the making and using of various embodiments of the presentinvention are discussed in detail below, it should be appreciated thatthe present invention provides many applicable inventive concepts, whichcan be embodied in a wide variety of specific contexts. The specificembodiments discussed herein are merely illustrative of specific ways tomake and use the invention and do not delimit the scope of theinvention.

FIG. 1 is a perspective view of a radar-absorbing panel having ahoneycomb structure and a lower skin assembly in which the individualcells of the honeycomb structure are fully filled with an aerogel inaccordance with one embodiment of the present invention. FIG. 2 is aclose-up perspective view of the honeycomb structure and lower skinassembly of FIG. 1. As seen in FIGS. 1 and 2, the honeycomb structure ismade of an array of individual cells. The cells preferably have ahexagonal cross-sectional area; however, it should be understood thatthe individual cells may have cross-sectional areas of differentgeometrical shapes. Also, the honeycomb structure may be formed fromcells having different cross-sectional shapes and sizes, depending uponthe effect desired. In addition, the individual cells may have differentcell geometries, including normally expanded, over expanded, underexpanded, and flex cell geometries.

The cells are filled with an aerogel in one or more forms, including agranular form. The aerogel may be pre-formed having a cross-sectionalshape that corresponds to the cross-sectional shape of the individualcells of the honeycomb structure, or the aerogel may be in a loosegranular form. For those applications in which the aerogel is ingranular form, the aerogel may be held together with a binder, thegrains may be free to move within the cells, or the grains may betightly packed within the cells. Additional advantages of filling thecells with an aerogel is that the cells can be made larger withoutsacrificing structural integrity, and reductions in the number of cells,typically leads to an overall reduction in the weight of the vehicle.

The type of aerogel used may vary by application. A wide range ofaerogels will be known to those of skill in the art. Specific examplesof suitable aerogels include silica, alumina, and zirconia aerogels. Theportion of each cell filled with aerogel may vary depending on theapplication. Selected individual cells of the honeycomb structure may befilled with aerogel using any of a number of processes, includingsifting, shaking, or raking of granular aerogel, as examples. Dependingupon the desired application, the honeycomb structure may be made of anyof a number of materials known to those of skill in the art. Thesematerials include, but are not limited to, Nomex, fiberglass, Kevlar,and Korex.

In certain applications, the cells may be filled partially with anaerogel and partially with an additional radar-absorbing and/or anadditional infrared-absorbing material. Although radar absorption isperformed by the material that forms the walls of the honeycomb, thismaterial is typically a poor thermal insulator. Partially filling thecells with a radar-absorbing material is advantageous because, by makingthe cells of the honeycomb layer larger and adding a radar absorbingmaterial to the aerogel, structural integrity is maintained, thermalconductivity is reduced, and radar absorption is maintained orincreased. For example, by adding graphite carbon to the aerogel, theradar absorbing properties of the panel can be considerably improved.Furthermore, it will be appreciated that a wide variety of materials maybe added to the aerogel to improve selected properties of the panel,such as electrical conductivity, thermal conductivity, radar absorption,and others. By selectively combining different materials in theindividual cells of the honeycomb structure, the overall properties ofthe panel can be selectively tuned for specific applications.

After the selected honeycomb cells are filled to the desired level withthe chosen combination of aerogel and/or other materials, an upper skinis added to the top of the honeycomb structure to complete the panel.The assembly is then cured. The skin material can vary from oneapplication to another. Examples of suitable materials includefiberglass, carbon fiber, and quartz. In certain applications usingcertain materials, a room temperature cure may be employed. Otherapplications may require elevated temperature and/or pressure in orderto effect a proper cure.

It has been determined that evacuation of the honeycomb cells providessignificant thermal advantages over the combination of aerogel and air.Alternately, the honeycomb cells can be filled with a low-density gas inorder to improve the thermal performance without the additionalmechanical stresses imposed by a pressure differential across the skins.

FIGS. 3 and 5 are perspective views of a radar-absorbing honeycombstructure and lower skin assembly having half of the individual cells ofthe honeycomb structure filled with aerogel and the other half empty inaccordance with one embodiment of the present invention. FIGS. 4 and 6are close-up perspective views of the honeycomb structure and lower skinassembly of FIGS. 3 and 5.

The aerogel-filled portion of the honeycomb assembly of FIGS. 3-6 issimilar to the aerogel-filled honeycomb assembly shown in FIGS. 1 and 2.The empty portion of the honeycomb assembly is distinct from thehoneycomb assembly of FIGS. 1 and 2 in that its cells are empty. Thisselective filling of certain individual cells of the honeycomb structureis particularly well suited for applications in which an infrared heatsource lies under a particular location of the panel, in that theaerogel-filled cells provide additional thermal insulation at thatlocation of the panel. In addition, this selective filling of certainindividual cells of the honeycomb structure is also beneficial inapplications in which a panel requires additional strength in a certainlocation. This is possible because packing individual cells with theaerogel and/or other materials adds strength to the panel.

FIG. 7 is a perspective view of a laboratory testing station useful fortesting the thermal performance of the panel of the present invention.FIG. 8 is a close-up perspective view of a blackbody heat source and atemperature controller shown in FIG. 7.

The testing station of FIGS. 7 and 8 incorporates a blackbody heatsource, a temperature controller, and one or more thermal camerasfocused on the surface of the blackbody heat source. The temperaturecontroller and blackbody heat source are designed work together tomaintain a uniform emission of infrared radiation from the front surfaceof the blackbody heat source. The thermal cameras are sensitive to theinfrared spectrum, rather than the visible spectrum, and can be used tocapture a thermal image of either the front surface of the blackbodyheat source or the front surface of an object disposed directly in frontof the blackbody heat source.

FIG. 9 is a perspective view of the laboratory testing station of FIG. 7showing a honeycomb panel according to the present invention affixed tothe front of the blackbody heat source. FIG. 10 is a close-upperspective view of the blackbody heat source and the temperaturecontroller shown in FIG. 9, with a honeycomb panel affixed to the frontof the blackbody heat source.

In this arrangement, the thermal cameras capture a thermal image of thefront surface of the honeycomb panel rather than the front surface ofthe blackbody heat source, as the honeycomb panel is disposed betweenthe blackbody heat source and the thermal cameras. Accordingly, owing tothe relatively uniform level of infrared radiation emitted from thefront surface of the blackbody heat source, this arrangement can be usedto measure the thermal characteristics of the honeycomb panel at variouspoints across its surface, at various temperatures, and at varioustimes.

FIG. 11 is a computer screen shot of a thermal image of the front sideof a honeycomb panel opposite the blackbody heat source obtained usingthe testing apparatus shown in FIGS. 9 and 10. It can be seen in FIG. 11that the temperature on the right half of the panel is considerablyhigher than the temperature on the left half of the panel. Thistemperature differential is attributable to the presence ofaerogel-filled cells on the left side of the honeycomb panel, ascontrasted with the empty cells on the right side of the panel.

FIG. 12 is a set of computer screen shots of thermal images of the frontside of a honeycomb panel having no aerogel in its cells. It can be seenin these figures that the temperature of the front side of the panelreaches a relatively steady state within approximately 18 minutes.

FIG. 13 is a set of computer screen shots of thermal images of the frontside of a honeycomb panel having its cells 25% filled with aerogel. Aswith the results shown in FIG. 12, it can be seen in these figures thatthe temperature of the front side of the panel reaches a relativelysteady state within approximately 18 minutes. In contrast to the resultsshown in FIG. 12, however, the results shown in FIG. 13 exhibitsubstantially lower temperatures than the results shown in FIG. 12.

FIG. 14 is a set of computer screen shots of thermal images of the frontside of a honeycomb panel having its cells 50% filled with aerogel. FIG.15 is a set of computer screen shots of thermal images of the front sideof a honeycomb panel having its cells 75% filled with aerogel. FIG. 16is a set of computer screen shots of thermal images of the front side ofa honeycomb panel having its cells 100% filled with aerogel. Comparisonof these results with one another and with the results shown in FIGS. 12and 13 reveals that the increase in the proportion of aerogel withineach cell has a substantial effect on the infrared signature of thepanel.

FIG. 17 is a set of computer screen shots of thermal images of the frontside of a honeycomb panel having half of its cells 100% filled withaerogel and half of its cells empty, in the same manner as the paneldescribed in connection with FIGS. 1-5 and 11. It can be seen in thisFigure that the thermal performance of this panel is similar to theperformance seen in FIG. 11.

It is apparent that an invention with significant advantages has beendescribed and illustrated. Although the present invention is shown in alimited number of forms, it is not limited to just these forms, but isamenable to various changes and modifications without departing from thespirit thereof.

1. A panel for a vehicle comprising: a lower skin; an upper skin; ahoneycomb structure formed from an array of individual cells havingselected cross-sectonal areas disposed between the inner skin and theouter skin; and a thermally insulative material disposed within theindividual cells of the honeycomb structure; wherein the honeycombstructure reduces a radar signature of the vehicle, and the thermallyinsulative material reduces an infrared signature of the vehicle.
 2. Thepanel according to claim 1, wherein the lower skin and the upper skinare formed from composite materials.
 3. The panel according to claim 2,wherein the lower skin is formed from a material selected from the groupof fiberglass, carbon fiber, and quartz.
 4. The panel according to claim2, wherein the upper skin is formed from a material selected from thegroup consisting of fiberglass, carbon fiber, and quartz.
 5. The panelaccording to claim 1, wherein the honeycomb structure is formed fromcomposite materials.
 6. The panel according to claim 5, wherein thehoneycomb structure is formed from a material selected from the groupconsisting of Nomex, fiberglass, Kevlar, and Korex.
 7. The panelaccording to claim 1, wherein the thermally insulative materialcompletely fills the individual cells of the honeycomb structure.
 8. Thepanel according to claim 1, wherein the thermally insulative materialpartially fills the individual cells of the honeycomb structure.
 9. Thepanel according to claim 1, wherein the thermally insulative material isan aerogel.
 10. The panel according to claim 9, wherein the aerogel ispre-formed to fit in the individual cells of the honeycomb structure.11. The panel according to claim 9, wherein the aerogel is from thegroup consisting of silica, alumina, and zirconia.
 12. The panelaccording to claim 9, wherein the aerogel is in a granular form.
 13. Thepanel according to claim 12, wherein the aerogel is held together with abinder.
 14. The panel according to claim 12, wherein the grains of theaerogel are free to move within the individual cells.
 15. The panelaccording to claim 12, wherein the grains of the aerogel are placed inthe individual cells by a sifting process.
 16. The panel according toclaim 12, wherein the grains of the aerogel are placed in the individualcells by a shaking process.
 17. The panel according to claim 12, whereinthe grains of the aerogel are placed in the individual cells by a rakingprocess.
 18. The panel according to claim 1, wherein the thermallyinsulative material is maintained in a vacuum.
 19. The panel accordingto claim 1, wherein the thermally insulative material is a combinationof an aerogel and a low-density gas.
 20. The panel according to claim 1,wherein the individual cells have a hexagon cross-sectional shape. 21.The panel according to claim 1, wherein the individual cells have anormally expanded geometry.
 22. The panel according to claim 1, whereinthe individual cells have an over expanded geometry.
 23. The panelaccording to claim 1, wherein the individual cells have an underexpanded geometry.
 24. The panel according to claim 1, wherein theindividual cells have an under expanded geometry.
 25. The panelaccording to claim 1, wherein the individual cells have a flex cellgeometry.
 26. The panel according to claim 1, further comprising: aradar absorbing material disposed within the individual cells of thehoneycomb structure; wherein the radar absorbing material additionallyreduces the radar signature of the vehicle.
 27. The panel according toclaim 1, wherein the radar absorbing material is graphite carbon.
 28. Apanel for a vehicle comprising: a lower skin; an upper skin; a honeycombstructure formed from an array of individual cells disposed between theinner skin and the outer skin; and a thermally insulative materialdisposed within selected individual cells of the honeycomb structure;wherein the honeycomb structure reduces a radar signature of thevehicle, and the thermally insulative material reduces an infraredsignature of the vehicle.
 29. A method of simultaneously reducing theradar signature and the infrared signature of a vehicle comprising thesteps of: providing a lower skin; providing an upper skin; forming ahoneycomb structure from an array of individual cells; disposing thehoneycomb structure between the inner skin and the outer skin; anddisposing a thermally insulative material within the individual cells ofthe honeycomb structure.
 30. The method according to claim 29, whereinthe step of disposing a thermally insulative material within theindividual cells of the honeycomb layer is achieved by completelyfilling the individual cells.
 31. The method according to claim 29,wherein the step of disposing a thermally insulative material within theindividual cells of the honeycomb layer is achieved by partially fillingthe individual cells.
 32. The method according to claim 29, wherein thethermally insulative material is an aerogel.
 33. The method according toclaim 29, wherein the thermally insulative material is a combination ofan aerogel and a low-density gas.
 34. The method according to claim 29,further comprising the step of: evacuating the individual cells afterthe insulative material is disposed therein.
 35. The method according toclaim 29, wherein the thermally insulative material is a combination ofan aerogel and a radar absorbing material.
 36. A method of selectivelytuning the infrared and radar absorbing properties of a vehicle panel,the method comprising the steps of: providing a lower skin; providing anupper skin; forming a honeycomb structure from an array of individualcells; disposing the honeycomb structure between the inner skin and theouter skin; and disposing a combination of selected materials within theindividual cells of the honeycomb structure to alter selected physicalproperties of the vehicle panel.